ENVIRONMENTAL AND POPULATION-SPECIFIC CONTRIBUTIONS TO GROWTH-RATE VARIATION IN THE MARINE AMPHIPOD JASSA-MARMORATA HOLMES

Local selective regimes and limited dispersal may contribute to geneti c differentiation of life history traits among local populations of th e marine amphipod Jassa marmorata Holmes. Reciprocal transplant experi ments were used to explore growth rate variation and to determine the effects of environment versus source population (genotype) on the obse rved variation. Gravid females were collected at two sites in Narragan sett Bay, Rhode Island. Hatchlings were measured and reared in field e nclosures following a split brood design. Measurements of total body s ize (length in mm) were made initially and at two additional time inte rvals. Experiments were conducted in spring and summer of 1994 and aga in in the spring of 1995. Growth curve analysis was used to test the e ffects of two independent variables, source population and environment , on growth rate. Results of the spring 1994 experiment revealed both source population (genetic) and environmental effects on growth rate. Individuals collected from and grown at Point Judith attained the high est body size. In the summer of 1994, there were no significant effect s of environment or source population on growth rates. In the spring o f 1995 only environmental effects were detected; animals grew faster a t Point Judith. These results suggest that divergence between populati ons may occur with respect to growth rate but this divergence is not c onsistent interannually. An alternative mechanism, phenotypic plastici ty, may also be employed by these amphipods to cope with environmental heterogeneity. Copyright (C) 1997 Elsevier Science B.V.